its: Add new preserveSpaceRule "paragraph"

its: Add new preserveSpaceRule "paragraph"

This implements a new text extraction rule preserving paragraph
boundaries, as mentioned in:
https://gitlab.gnome.org/GNOME/glib/issues/1350

* gettext-tools/doc/gettext.texi (Preparing ITS Rules): Mention "paragraph".
* gettext-tools/src/its.c (its_rule_list_extract_text): Accept "paragraph".
(its_merge_context_merge_node): Likewise.
(normalize_whitespace): Handle "paragraph" rule.
* gettext-tools/src/its.h (ITS_WHITESPACE_NORMALIZE_PARAGRAPH): New enum value.
* gettext-tools/tests/xgettext-its-1: Add test for "paragraph" rule.